Processing Raw Biogas Into Renewable Natural Gas Suitable For Pipeline Blending

By: Alice Fu, Senior Global Product Manager, Emerson

In these days of growing efforts to reduce the effects of climate change, the agriculture sector is often cited as a major source of greenhouse gases (GHGs), including methane (CH4) and nitrous oxide (N2O), with those two sources combined accounting for about 10% of global non-carbon dioxide (CO2) emissions. Much of this comes from manure, but also from crop waste if left to deteriorate in the open. Using anerobic digestion in an enclosed system to process crop waste (Figure 1) allows collection of the resulting biogas, and its use of CH4, as a fuel source, while preventing its release to atmosphere.

Capturing biogas is not a new concept and has been used extensively by large agricultural sites as a means to deliver essentially free fuel as a byproduct of normal waste processing. Generally, this biogas has been used within a given facility to power equipment suited to handling minimally processed gas, such as boilers or engine-generator sets.

But this raw biogas right out of the digester gas can be refined to the extent necessary for it to be used as renewable natural gas (RNG, aka biomethane in some regions) such that it can be added to pipelines interchangeably with the traditional fossil product, reducing consumption of fossil sources. Let’s take a closer look at what this requires.

Understanding Biogas Composition

Anerobic digestion is fermentation of organic matter by bacteria. It is similar to natural biodegradation but happens more quickly in a reactor in the absence of oxygen, and this process allows for capturing the gaseous products. This is neither a clean nor precise reaction due to the wide range of variabilities, but still fairly predictable. Biogas normally contains:

Obviously, raw biogas is a long way from qualifying as sales gas carried by pipelines and utilities. Moreover, a pipeline company accepting gas from a source such as a biogas producer will insist on having it fall within normal compositional limitations to avoid contaminants or ballast gases that reduce overall BTU value. So how much does it need to be cleaned up to meet basic requirements for sales gas?

Regulatory agencies and pipeline companies around the world define natural gas quality by limits on components other than CH4. While there is some minor variability, most regions include specification ranges for:

Regulations are tight as variability in sales gas can create a variety of problems for large-scale users, particularly when used as a fuel for gas turbines or sophisticated fired heaters, where the BTU rating is critical. For gas turbines, the Wobbe Index indicates additional characteristics affecting its suitability as a fuel. And for some users, contaminants can be just as important. For most, the least desirable contaminants relate to sulfur, which influences the price and quality of gas being transferred.

Pipeline operators must test sales gas at various points in the distribution chain to ensure it has correct composition. Various testing methods are available, as we will discuss in a moment.

Having said all that, what is the practicality of converting biogas into RNG, within all the limitations of pipeline regulations?

Cleaning Up Biogas

Undertaking such a project must be driven by a desire to reduce fossil fuel use. RNG is considered carbon-neutral because the CO2 released when burned comes from the CO2 that was originally absorbed by the plants during photosynthesis, essentially creating a closed carbon cycle where no net new carbon is added to the atmosphere.

Conventional natural gas is not considered carbon neutral because, despite producing less CO2 than other fossil fuels when burned, it is primarily composed of CH4, a potent GHG. Throughout natural gas production fields, CH4 leaks significantly throughout its extraction, transportation, and distribution processes, negating some of its climate benefit. CH4 leaks contribute significantly to overall GHG effects from natural gas, at rates more than CO2.

The process of converting biogas into RNG, also known as biogas upgrading, requires the removal of unwanted components—including CO2, H2S, and H2O—from raw biogas, leaving behind a much higher concentration of CH4 that can be injected into the natural gas grid. Common upgrading technologies include:

The selection of each technology depends on the desired purity level and project volume requirements, and in some circumstances may require more than one removal technique, depending on the gas composition. The aim of upgrading technologies is to achieve the highest practical CH4 purity with minimum energy consumption and overall volume loss. Examining the pros and cons of various options is beyond the scope of this article, but all options require a method to measure final gas composition, and possibly at additional intermediate points.

Gas Chromatography for Composition Analysis

Gas chromatography (GC) technology is firmly established as a recognized method for composition analysis of natural gas at production and custody transfer points due to its high reliability, measurement accuracy, and minimal downtime. Tens of thousands of GC analyzers are installed worldwide along natural gas pipelines, and due to the similarity of biogas measurement with natural gas measurement, GC analyzers are also trusted by biogas producers, transporters, and users.

A GC analyzer functions by separating and detecting chemical components within a sample. Inside the analytical oven, a column facilitates separation of various components, while valves inject the sample and direct it through different flow paths. Finally, detectors are employed to identify and quantify the separated sample components.

A GC analyzer utilizes different types of detection technologies depending on the gas composition. The most common techniques are thermal conductivity detector (TCD), flame ionization detector (FID), and flame photometric detector (FPD). TCDs are for universal detection, mainly to measure inert gases and most hydrocarbons, while FIDs are adept at quantifying trace hydrocarbons, and FPDs specialize in measuring low-level sulfur species.

With speciated composition measurement, GC software can calculate many critical physical properties of the gas according to the latest industry standards, such as BTU value, Wobbe index, specific gravity, compressibility, etc. For characterizing biogas and monitoring upgrading operations, TCD detectors are generally sufficient since there are relatively few sulfur compounds present and few higher hydrocarbons.

GC Analyzer Operation

The gas sample under test is mixed with an inert carrier gas and passes through a packed column (Figure 2). During the stationary phase in the column, the components separate and exit the column in a prescribed order, allowing them to be identified and quantified using TCD.

The column and TCD are inside an oven to maintain the required temperature (Figure 3).

Signal processing by specialized software embedded in the GC analyzer determines which individual chemical components are present and in what proportions. These are indicated by peaks on the graph (Figure 4).

GC analyzer suppliers design their instruments to fit the list of components that end users demand for their routine applications. Some instruments cover a longer list of components than others, and this becomes a key specification, driving price and operational complexity. For biogas or conventional natural gas analysis, a basic GC analyzer can typically handle hydrocarbons through hexanes, although hydrocarbons higher than CH4 in biogas are usually only present in trace amounts.

Selecting a GC Analyzer

RNG measurement standards, where it is used as a stand-alone fuel, are still being developed in the different world areas and can vary from country to country, and even from contract to contract. However, where RNG must be interchangeable with conventional pipeline natural gas, the measurement requirements are the same as with sales gas analysis.

Whichever the case, sites that produce biogas generally do it as a sideline, with their main operations focused on dairy products or meat production. Consequently, biogas is decidedly secondary, so selecting an analyzer technology will hinge on minimizing long-term operational cost and providing overall ease of use.

Fortunately, today’s GC analyzers aren’t the complex, fragile, bulky, and expensive devices of yesteryear. The need for consumables is much lower, and improved operator interfaces allow automated processing, remote connectivity, and audit checks—while reducing the need for specialized operator training. The smaller size and greater ruggedness of internal valves and sensors makes it possible to reduce the footprint of a GC analyzer, while eliminating the need for specialized enclosures (Figure 5).

Most biogas producers choose a GC analyzer with a single TCD, such as Emerson’s Rosemount™ 470XA Gas Chromatograph, due to its cost effectiveness. Others may prefer a GC analyzer with two TCDs, such as Emerson’s Rosemount 770XA Gas Chromatograph, to achieve expanded measurement into higher hydrocarbons, along with low ppm-level H2S. The latter unit also delivers a shorter cycle time. Both designs are explosion-proof and NEC/ ATEX/IECEX safety rated—so they are field installable without an enclosure—even in Class 1, Division 1 areas, eliminating the need for costly air-purged enclosures.

RNG Implementation Drivers

The processing steps necessary to turn raw biogas into RNG results in a production cost between five and 10 times the cost of conventional natural gas. The primary factor is the cost of purification via removal of CO2 and sulfur compounds, as these are expensive and complex processes. The analyzer itself is no different than would be necessary for a conventional natural gas production site delivering product to a pipeline operator.

Therefore, the fundamental drivers behind RNG production are government mandates and initiatives aimed at lowering GHG emissions. For the producers to be profitable, or at least minimize required subsidies, there is a strong need for cost effective measurement solutions. For dirty raw gas, a robust analyzer solution capable of measuring H2S as well as BTU content in one single-analyzer solution is thus highly desired.

Emerson’s GC analyzer offerings are ideal for these applications, delivering accurate measurements with simple operation and low lifetime costs. Biogas producers, pipeline companies, and end users get the measurements they need without the complications, expense, and maintenance requirements of older technologies and solutions.

About the Author

Alice Fu is a Senior Global Product Manager at Emerson, supporting the Rosemount Gas Chromatograph product line.

She has been with Emerson for the past 20 years in multiple roles, including customer support, analytical system integration, project management, business leadership, and currently product management.

Fu holds a bachelor’s degree in biomedical engineering from Zhejiang University, and MBA from Shanghai Jiao Tong University, both in China.